Conventional hard-tissue implants include implants designed to promote in-growth of hard tissue based on forming a tissue/implant interface in which the implant forms a continuous phase and the tissue forms a discontinuous phase, e.g. based on the implant having a concave and/or porous surface into which the hard tissue can grow, and designed to have add-on surface modifications, e.g. modifications added based on sintering.
For example, Van Kampen et al., U.S. Pat. No. 4,608,052, discloses an implant for use in a human body having an integral attachment surface adapted to permit ingrowth of living tissue. The implant surface is defined by a multiplicity of adjacent, generally concave surface parts having intersecting, generally aligned rims defining an inner attachment surface portion and by a multiplicity of spaced posts projecting from the inner attachment surface. Van Kampen also discloses that implants have been provided with porous surfaces, as described in U.S. Pat. Nos. 3,605,123, 3,808,606, and 3,855,638.
Also for example, J. D. Bobyn et al, 150 Clinical Orthopaedics & Related Research 263 (1980), discloses that a pore size range of approximately 50 to 400 μm provided an optimal or maximal fixation strength (17 MPa) in the shortest time period (8 weeks) with regard to cobalt-base alloy implants with powder-made porous surfaces. Specifically, implants were fabricated based on coating cylindrical rods of cast cobalt-base alloy with cobalt base alloy powder in four particle size ranges. The particle size ranges were as follows: 25 to 45 μm; 45 to 150 μm; 150 to 300 μm; and 300 to 840 μm. The corresponding pore size ranges of the particles were as follows: 20 to 50 μm; 50 to 200 μm; 200 to 400 μm; and 400 to 800 μm, respectively. The particles were then bonded to the rods based on sintering. All implants were manufactured to have a maximal diameter of 4.5 mm and a length of 9.0 mm. The implants were surgically inserted into holes in dog femurs and bone ingrowth was allowed to proceed. After varying periods of time (4, 8, or 12 weeks), the maximum force required to dislodge the implants was determined. Implants with a pore size lower than 50 μm yielded relatively low fixation strengths at all time points, while implants with a pore size higher than 400 μm exhibited relatively high scatter with regard to fixation strengths, thus indicating that a pore size range of approximately 50 to 400 μm provided an optimal or maximal fixation strength.
Conventional hard-tissue implants also include implants having surface texturing, e.g. barbs or pillars, to make it difficult to withdraw the implants from hard tissue or to more effectively mechanically anchor at an early date or affix into adjoining hard tissue.
For example, Amrich et al., U.S. Pat. No. 7,018,418, discloses implants having a textured surface with microrecesses such that the outer surface overhangs the microrecesses. In one embodiment, unidirectional barbs are produced in the surface that can be inserted into bone or tissue. The directional orientation of the barbs is intended to make it difficult to withdraw from the bone or tissue.
Also for example, Picha, U.S. Pat. No. 7,556,648, discloses a spinal implant, i.e. an implant for use in fusing and stabilizing adjoining spinal vertebrae, including a hollow, generally tubular shell having an exterior lateral surface, a leading end, and a trailing end. The exterior surface includes a plurality of pillars arranged in a non-helical array. Each pillar has a height of 100 to 4,500 μm and a lateral dimension at the widest point of 100 to 4,500 μm. The exterior surface also has a plurality of holes therethrough to permit bone ingrowth therethrough.
Unfortunately, interfaces of hard tissue and hard-tissue implants in which the hard tissue is in a discontinuous phase may be susceptible to stress shielding, resulting in resorption of affected hard tissue, e.g. bone resorption, over time. Also, addition of surface texturing to implants by sintering can result in the surface texturing occupying an excessive volume of corresponding hard tissue/implant interfaces, leaving insufficient space for hard tissue. In addition, spinal implants are designed to perform under conditions relevant to spine, i.e. compression, rotational shear, and vertical shear, with the compression being essentially constant, the rotational shear being intermittent, and the vertical shear being rare, rather than conditions relevant to other hard tissues such as long bone, maxillary bone, mandibular bone, and membranous bone, i.e. load bearing conditions, including compression and tension, varying across the hard tissue and across time, and intermittent rotational and vertical shear. Accordingly, there is a need for hard-tissue implants of general applicability that address these issues and provide improvements. The device disclosed here is such an implant.